Siren is cool because she can play however she wants honestly.
She has the tools to play heavy creature swarm or small-forces, agro beatdown, attack-spell/ranged centric, multi source casting, position control. There really is no "this is what you do" with her, which is why she is instantly one of my favorite and in my opinion, one of the best mages.
Also in team play, Siren+Druid is a savage team build. (friendly hydro-immune plants+whirlpools and tsunamis!)